The invention relates to a process for making an ultra-high-molecular-weight polyethylene (UHMWPE). More particularly, the invention relates to a process for making an UHMWPE with a supported transition metal catalyst having at least one quinolinoxy ligand.
Ultra-high-molecular-weight polyethylene (UHMWPE) has a molecular weight that is 10 to 20 times greater than high-density polyethylene (HDPE). It has been defined by ASTM as having a weight average molecular weight (Mw) greater than 3,000,000. In addition to the chemical resistance, lubricity, and excellent electrical properties of conventional HDPE, UHMWPE offers major advantages in toughness, abrasion resistance, and freedom from stress-cracking.
UHMWPE is produced by Ziegler polymerization. For example, U.S. Pat. No. 5,756,600 teaches a process for making UHMWPE with Ziegler catalysts. The process requires exceptionally pure ethylene and other raw materials. An xcex1-olefin comonomer, such as 1-butene, may be incorporated into UHMWPE according to U.S. Pat. No. 5,756,600. Like conventional HDPE, UHMWPE made by Ziegler polymerization has a broad molecular weight distribution.
Newly developed single-site catalysts advantageously provide polyethylene and other polyolefins with narrow molecular weight distribution (Mw/Mn from 1 to 5). The narrow molecular weight distribution is a reflection of reduced low molecular weight species. These new catalysts also significantly enhance incorporation of long-chain xcex1-olefin comonomers into polyethylene, and therefore reduce its density.
It is difficult to produce UHMWPE with single-site catalysts. For example, U.S. Pat. No. 5,444,145 teaches preparation of polyethylene having a weight average molecular weight up to 1,000,000 with a cyclopentadienyl-based single-site catalyst. However, its molecular weight is significantly lower than that required for UHMWPE.
U.S. Pat. No. 6,265,504 teaches a process for making an UHMWPE with an unsupported heteroatomic ligand-containing single-site catalyst. The process, however, has low catalyst activity and the UHMWPE produced has relatively low tensile and impact properties.
A new process for making UHMWPE is needed. Ideally, the process would give high catalyst activity and produce an UHMWPE having improved tensile and impact properties.
The invention is a process for preparing an ultra-high-molecular-weight polyethylene (UHMWPE). The process comprises supporting a single-site catalyst comprising a Group 3-10 transition or lanthanide metal and a quinolinoxy ligand onto a support and polymerizing ethylene in the presence of the supported catalyst and a non-alumoxane activator. The polymerization is performed at a temperature within the range of about 40xc2x0 C. to about 110xc2x0 C. in the absence of an aromatic solvent, an xcex1-olefin comonomer, and hydrogen.
The process of the invention has high catalyst activity and produces UHMWPE that has improved tensile properties and impact resistance.
The invention is a process for making an ultra-high-molecular-weight polyethylene (UHMWPE). The process comprises supporting a single-site catalyst comprising a Group 3-10 transition or lanthanide metal and a quinolinoxy ligand onto a support. Preferably, the single-site catalyst comprises a Group 4 transition metal. More preferably, the transition metal is titanium or zirconium.
The total number of ligands satisfies the valence of the transition metal. Other suitable ligands include substituted or unsubstituted cyclopentadienyls, indenyls, and fluorenyls, halides, C1-C10 alkyls, C6-C15 aryls, C7-C20 aralkyls, dialkylamino, thioether, siloxy, alkoxy, and the like, and mixtures thereof. Benzyl, halide, cyclopentadienyl, and indenyl ligands are preferred. Benzyl ligands are particularly preferred. More preferably, the transition metal catalyst contains one quinolinoxy and three benzyl ligands.
The support is preferably a porous material such as inorganic oxides and chlorides, and organic polymer resins. Preferred inorganic oxides include oxides of Group 2, 3, 4, 5, 13, or 14 elements. Preferred supports include silica, alumina, silica-aluminas, magnesias, titanias, zirconias, magnesium chloride, and crosslinked polystyrene. Silica is most preferred.
Preferably, the support has a surface area in the range of about 10 to about 900 m2/g, a pore volume in the range of about 0.1 to about 4.0 mL/g, an average particle size in the range of about 10 to about 500 xcexcm, and an average pore diameter in the range of about 10 to about 1000 xc3x85. The support is preferably modified by heat treatment, chemical modification, or both. For heat treatment, the support is preferably heated at a temperature from about 50xc2x0 C. to about 800xc2x0 C. More preferably, the temperature is from about 100xc2x0 C. to about 400xc2x0 C.
Suitable chemical modifiers include organoaluminum, organosilicon, organomagnesium, and organoboron compounds. Organosilicon and organoboron compounds, such as hexamethyldisilazane (HMDS) and triethylborane, are preferred. Suitable techniques for treating a support are taught, for example, by U.S. Pat. No. 6,211,311, the teachings of which are incorporated herein by reference.
Preferably, the supporting involves treating a support with organosilicon compounds, calcining the treated support, treating the calcined support with organomagnesium compounds, mixing the organomagnesium-treated support with a quinolinoxy ligand-containing single-site catalyst, and then removing any solvents from the supported catalyst. More preferably, the supporting is performed by (1) treating a silica support with HMDS, (2) calcining the HMDS-treated silica (3) treating the calcined silica with dibutylmagnesium, (4) mixing the treated silica of step 3 with a quinolinoxy ligand-containing single-site catalyst, and (5) removing any solvents. Example 1 shows a detailed procedure of supporting the catalyst.
Other suitable supporting techniques may be used. For example, the catalyst may be supported by using the method taught by co-pending application Ser. No. 09/781,464. First, a quinolinol is deprotonated to produce an anionic ligand precursor. Second, the anionic ligand precursor reacts with about 0.5 equivalent of a transition metal compound to give a mixture that contains quinolinoxy ligand-containing catalyst. Third, the mixture reacts with a non-alumoxane activator. Fourth, the product from step three is combined with a support. Finally, the solvents are removed to give a solid, supported catalyst.
Suitable non-alumoxane activators include alkyl aluminums, alkyl aluminum halides, anionic compounds of boron or aluminum, trialkylboron and triarylboron compounds, and the like. Examples are triethylaluminum, trimethylaluminum, diethylaluminum chloride, lithium tetrakis(pentafluorophenyl) borate, triphenylcarbenium tetrakis(pentafluorophenyl) borate, lithium tetrakis(pentafluorophenyl) aluminate, tris(pentafluorophenyl) boron, tris(pentabromophenyl) boron, and the like. Other suitable activators are known, for example, in U.S. Pat. Nos. 5,756,611, 5,064,802, and 5,599,761, and their teachings are incorporated herein by reference. Alumoxane compounds, such as methyl alumoxane or ethyl alumoxane, are not suitable activators for the process of the invention. When an alumoxane activator is used, UHMWPE cannot be made.
Activators are generally used in an amount within the range of about 0.01 to about 100,000, preferably from about 0.1 to about 1,000, and most preferably from about 0.5 to about 50, moles per mole of the catalyst.
The polymerization is conducted at a temperature within the range of about 40xc2x0 C. to 110xc2x0 C., preferably about 50xc2x0 C. to 80xc2x0 C. A high polymerization temperature results in a low molecular weight of polyethylene. If the temperature is too high, UHMWPE cannot be obtained.
The polymerization is preferably conducted under pressure. The reactor pressure is preferably within the range of about 100 to about 5,000 psi, more preferably from about 300 to about 3,000 psi, and most preferably from about 500 to about 2,000 psi. Generally, the higher the pressure, the more productive the process.
The process of the invention includes slurry and gas phase polymerizations. The process is conducted in the absence of an aromatic solvent. Saturated aliphatic and cyclic hydrocarbons are suitable solvents. Preferred solvents include pentane, hexane, heptane, octane, isobutane, cyclohexane, and the like, and mixtures thereof. Using an aromatic solvent in the process reduces the molecular weight of polyethylene. UHMWPE cannot be obtained when an aromatic solvent is used.
The process of the invention is performed in the absence of hydrogen or any other chain transfer agent. Using hydrogen in the process reduces the molecular weight of the polyethylene. UHMWPE cannot be obtained in the presence of hydrogen.
The process of the invention is conducted in the absence of other xcex1-olefin comonomers such as propylene, 1-butene, or 1-hexene. Incorporation of an xcex1-olefin comonomer reduces the molecular weight of polyethylene. UHMWPE cannot be obtained when an xcex1-olefin comonomer is used.
I have surprisingly found that the process of the invention gives much higher catalyst activity than the known process (see Table 1). For instance, when 8-quinolinoxytitanium tribenzyl is supported (Example 3), the catalyst activity is more than 6 times greater than that when the catalyst is not supported (Comparative Example 4). More surprisingly, UHMWPE produced by the process of invention shows significantly improved tensile properties and impact resistance (see Table 2).
UHMWPE made by the process of the invention has a Mw that is greater than about 3,000,000 and Mw/Mn less than about 5. Preferably, Mw is greater than about 4,500,000 and Mw/Mn is less than about 3. UHMWPE has a variety of uses. In particular, it can be advantageously used to make film, pressure pipe, large-part blow molding, extruded sheet, and many other articles. It can be used alone or blended with other resins. Techniques for making these articles are well known in the polyolefin industry.
The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.